FIELD OF THE INVENTION
This invention relates in general to the construction of sewing machines and in particular to a combination sewing machine and jig therefor for holding the sewing machine for machining purposes.
The invention is concerned particularly with the construction of sewing machine housings such that they may be held in clamping jigs by clamping elements which do not mar or destroy the finish thereof.
In the prior art, such housings are machined in unfinished state, prior to applying the final lacquer coat. To this end, the housing is held fast in a clamping fixture, with the clamping elements applied preferably to locations at opposite outsides of the housing. At these locations, however, the clamping elements leave impressions which, prior to putting on the final lacquer coat, must be filled out and then ground smooth.
To minimize the labor costs for removing the lacquer residues, from the machined surfaces, threads, and bores after the final lacquer coating, these machined areas are masked prior to the finishing as far as possible. This requires a plurality of various types of masking elements which must then be cleaned and replaced after relatively short time periods and with considerable expenses.
Certain areas of the housing, such as the guide for the oscillating needle bar, the bore for the presser foot bar, and screw-on surfaces for the bed plate, cannot be satisfactorily covered to avoid lacquer penetration, and must be cleaned manually after the final coating.
During the lacquer drying operation, the finished housings are exposed to relatively high temperatures. This produces stresses in the material which not infrequently lead to such warpage of the housings that the dimensions obtained with the machining are changed. Particularly frequent is a misaligning of the bearing bores initially aligned in a common straight line. To mitigate this result affecting the quality, the housings have been subjected, prior to the machining, to a heat treatment to remove the stresses in the material. This, however did not lead to any substantial improvement.